Is my job as an accountant at a company that sells, installs, and maintains sound systems for dance halls considered unlawful?

Your work in the company is permissible because it does not specialize in selling and installing loudspeakers for unlawful dance halls. You must advise those in charge of the company not to sell loudspeakers to such halls, in accordance with the Almighty's saying: "And cooperate not in sin and aggression."